This time I will show you the small changes which will appear in every migrated APEX application.

For the better understanding I show you an old and a new example (all tested with Mac and Firefox):

Date Picker - Icon:

Date Picker - Date Selector:

Rich Text Editor:

Interactive Report (IRR):
Compared to the other both examples the new IRR makes the biggest different. For that I added 3 examples with different themes to show you what you have to expect.
As Joel Kallman already mentioned the IRR will not work with custom plugins or changes as it did before. None of my examples uses custom plugins or JS code. Only CSS code has maybe been modified!

Example 1 - Sample Application

Example 2

Example 3

So be aware that especially the interactive report does not look like before. You may even have to consider more work in updating your application documentation.